1. Field of the Invention
This inventions relates to hammers for hammer mills. The hammers are especially intended for use in the hammer mills of tub grinders or tumble grinders.
U.S. Pat. No. 5,181,663 granted to the present inventors in 1993 provides an example of a tub or "tumble" grinder which includes a hammer mill. Such grinders have a receptacle with a stationary floor and a rotatable cylindrical wall. The axis of rotation of the receptacle may be vertical (as is conventional), or preferably angled as described and illustrated in the patent. The hammer mill is positioned under the floor of the receptacle, and the hammers extend partially through an aperture in the floor. As the receptacle rotates with the hammer mill in operation, the hammers pull debris from the receptacle through the aperture into a grinding chamber. The debris is fragmented and sized in passing through the grinding chamber, and is then discharged. Ideally, the fragments are relatively uniform in size, although this is difficult to achieve in practice.
2. Description of the Prior Art
Grinders such as the one described in the above-mentioned patent, and others in the prior art, conventionally have a series of pivotable bar hammers, i.e. rectangular bars which are pivotally mounted on the mill. These bar hammers have a relatively small mass, typically about 28 pounds for example, and provide essentially a single impact, so they tend not to break up the debris very much on initial impact. Most of the reduction or "sizing" of the debris takes place in passing through the grinding chamber. Because of their relatively small mass, these hammers also tend to pivot or "lay back" on impact with the debris, instead of maintaining an orientation directly facing the debris. This significantly reduces the efficiency and effectiveness of the hammers, since they then only strike a glancing blow, and also results in less consistent sizing of the processed debris, since the tip clearance within the mill is increased as the hammers lay back.
Conventional bar hammers also become less efficient and effective as they wear, since the initially-square edges become rounded. This reduced effectiveness also increases the tendency of the hammers to lay back, thus further reducing efficiency. The hammers must therefore be replaced more frequently than is desirable, in order to maintain optimum efficiency and effectiveness.